Doxorubicin

Major Uses

Doxorubicin is an anthrocycline anticancer drug used as a chemotherapy agent for dogs and cats. It is most commonly used to treat osteosarcoma, other sarcomas, lymphoma, carcinomas, and melanoma, and it has been used to treat small animal cancers for many years. This medication may be used alone or in combination with other treatments or chemotherapy drugs.

Doxorubicin treats cancer by impairing DNA synthesis in rapidly dividing cells. It is not approved by the Food and Drug Administration for use in animals, but it is legally prescribed as an extra-label medication by veterinarians.

Common Precautions

Doxorubicin is known to cause a number of potentially serious side effects in animals, including fever, hair loss, vomiting, diarrhea, loss of appetite, low blood cell counts, oral inflammation, damage to the heart muscle, abnormal heart rhythms, and kidney damage. Bone marrow toxicities are another risk and are common with most chemotherapy drugs. Additionally, doxorubicin can cause severe tissue damage if leaked from a vein.

Due to its cardiac toxic effects, animals undergoing treatment with doxorubicin require close monitoring. Regular blood counts are necessary, and the risk of systemic infection increases as bone marrow is suppressed. Acute allergic reactions (vomiting, swelling, hives, heart rhythm problems) are common; pretreatment with diphenhydramine is beneficial.

Certain breeds with a specific mutated gene, called the Multi Drug Resistant 1 (MDR1) gene, have a defective P-glycoprotein and are unable to pump doxorubicin from cells, resulting in toxicity. Other breeds, including Doberman Pinschers, Great Danes, and Boxers, are genetically predisposed to a condition called dilative cardiomyopathy that makes them more vulnerable to the cardiotoxic side effects of doxorubicin.

Because doxorubicin is activated in the liver, animals with liver disease may be unable to activate this medication and be unresponsive to its effects. Doxorubicin is contraindicated for use during pregnancy.

Medications that increase the risk for doxorubicin toxicity include cyclosporine, calcium channel blockers, and other chemotherapy agents. Medications known to decrease the effectiveness of doxorubicin include glucosamine and phenobarbital.

Doxorubicin should not be used in animals with a known allergy or hypersensitivity to the drug. There is a maximum amount of doxorubicin that can be given to an individual animal during its lifetime before cardiotoxicity occurs. Any indication of acute or chronic overdose should be reported to a veterinarian right away.

Storage

Doxorubicin is a potent medication and should always be stored in its original container, far out of the reach of children and pets.

Administration

Doxorubicin is given as an intravenous injectable over approximately 10 minutes in an IV drip. Extreme care must be taken to keep the medication off the skin and to prevent leakage into tissue surrounding the vein.

A typical dose for dogs is 30 mg/m2 every 21 days, depending on the exact diagnosis. A typical dose for cats is 20 to 30 mg/m2 every three to four weeks. Duration of administration depends on individual response to the medication, the protocol used, and the development of adverse effects. Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely.

Dogs should be pretreated with 1mg/lb IM of diphenhydramine or another H1 antihistamine. Cats do not require pretreatment with an antihistamine.

If a dose is missed or incomplete, a veterinarian should be consulted for advice. Two doses of this medication should never be administered at the same time.